Hi Alex,
In addition to potential attention from law enforcement agencies, keep in mind that running a Tor exit from your home connection (assuming you aren't able to get a second dedicated IP for the relay) is likely to significantly impact your regular Internet use. Some service providers either block exit relays or make people connecting from an exit IP jump through additional hoops (e.g., Cloudflare). You've probably noticed some of these issues when browsing with Tor - browsing from the same IP as an exit relay will be pretty similar (apart from speed, naturally).
Would you guys suggest running a bridge/relay instead, or just not helping out in terms of bandwidth contribution?
Running a bridge or non-exit relay from a home connection is fine - I haven't done it myself, but others on the list do.
Regarding throughput on a Raspberry Pi: as others have said, the Pi won't saturate a 100Mb connection. If you're just after something that's cheap and Pi-sized, you'd probably have better luck with something like the ODROID C1[0]; you still may not saturate the link though.
Regards, Sharif
[0] http://www.hardkernel.com/main/products/prdt_info.php?g_code=G143703355573